1. Field of the Invention
The present invention relates to an optical information recording and reproduction apparatus for recording information in an optical recording medium, such as an optical disk, or reproducing the recorded information from the optical recording medium, and, particularly, to the rationalization of a correction technique for correcting spherical aberration of an optical recording medium.
2. Related Background Art
In recent years, in the case of an optical disk apparatus, a technique corresponding to a change of making the wavelength of a light source shorter and a change of making the numerical aperture (NA) of an objective lens higher is vigorously studied, in order to make the density of a disk higher. Locally, commercialization of an apparatus using a 405 nm-band semiconductor laser and an objective lens of NA=0.85 is started.
Moreover, in order to make the capacity of a disk higher, a disk having two recording layers is used. However, when using the 405 nm-band semiconductor laser, its quantization noise, so-called laser diode (LD) noise, becomes a problem and, when using the objective lens of NA=0.85, spherical aberration due to error in the thickness of a transparent substrate or a jump operation between the layers of the double-layer disk becomes a problem.
Furthermore, the spherical aberration due to a jump operation between the layers cannot be ignored for a DVD of a red LD and an NA (numerical aperture) of about 0.65.
Therefore, when increasing the output power, because of the characteristic of a semiconductor laser (hereafter abbreviated as “LD”), LD noises are lowered. Accordingly, a technique for restraining LD noises by inserting an optical attenuation device, when a single-layer disk is used, is disclosed in Japanese Patent Application Laid-Open No. 2003-257072.
Moreover, a technique for generating a spherical aberration, by additionally using a beam expander and changing an interval between lenses to offset the spherical aberration due to a thickness error of a transparent substrate, is disclosed in Japanese Patent Application Laid-Open No. 2002-236252.
However, the above conventional techniques have the following problems.
That is, when inserting an optical filter between an LD and a collimator lens, this is effective for making an optical system compact, but an optical path length between the LD and the collimator lens depends on the presence or the absence of the optical filter. As a result, a case in which an optical filter is assumed as the reference stage of the optical path length between the LD and the collimator lens, the LD is present at a defocus position viewed from the collimator lens when the optical filter is not inserted. Then, the light flux after the collimator lens becomes convergent light, and when condensing the light to a disk by the optical lens, a large spherical aberration occurs.
However, a spherical aberration also occurs due to a thickness error of a transparent substrate of a disk or a thickness difference between layers. Therefore, by using spherical aberration correcting means, and searching the optimum state of the spherical aberration correcting means so that a signal quality serving as an index-signal becomes preferable, it is possible to correct spherical aberration occurring as a result.
Therefore, when searching the optimum state of the spherical aberration correcting means (hereafter, the searching of the optimum state of the spherical aberration correcting means is referred to as “spherical aberration learning”), the optimum state is searched by using an initial state serving as a reference as a start point. However, as described above, particularly, in the case of using an optical filter, the initial state is changed due to the presence or the absence of the optical filter. Therefore, in the case of spherical aberration learning, the range of the learning expands by at least a change of the initial state, and wasting of time of spherical aberration learning occurs.
Moreover, even if a system between an LD and a collimator lens is not changed, or even in the case of an apparatus capable of recording information on and reproducing information from a disk formed in accordance with a different format, for example, use of wavelengths different from each other, because the format differs, a spherical aberration amount to be corrected also differs. Therefore, it is necessary to widely set a learning range for spherical aberration learning, and by learning spherical aberration in accordance with the set range, a waste of learning time occurs.